Rills caused by runoff concentration on erodible hillslopes generally have very irregular longitudinal profiles and cross-section shapes. Rill erosion directly depends on the hydraulics of flow within the rills which may differ greatly from that in larger and regular channels like streams or rivers. At first, in this paper, a review of the two different approaches to estimate rill flow velocity, based on flow regime and flow resistance laws, is presented.

Recent advances in measurements of rill geometry by the three-dimensional photo-reconstruction (3D-PR) technique, which allows one to obtain a Digital Terrain Model (DTM) by low-altitude aerial and terrestrial photogrammetry, are also discussed. Indeed, information on rill geometry is also necessary to estimate flow velocity and depth using flow resistance equations.

Then, the results of related laboratory and field investigations carried out from 1984 to 2022 are reported. In particular, the review was developed considering the studies conducted investigating the different components of rill flow resistance due to grain resistance, morphological resistance (step-pool), and sediment transport. Moreover, a brief mention of the slope threshold, equal to 18%, determining differences in hydraulic and sediment transport variables, is done.

Also, an analysis is developed to demonstrate that a theoretical rill flow resistance law has been erroneously presented as an example of spurious correlation.

Finally, some research needs on rill flow hydraulics are highlighted.

由易蚀山坡上的径流集中引起的溪流通常具有非常不规则的纵向剖面和横截面形状。小溪侵蚀直接取决于小溪内的水力流动，这可能与河流或河流等较大且规则的渠道中的水力有很大不同。首先，在本文中，回顾了基于流态和流动阻力定律来估计小溪流速的两种不同方法。

还讨论了通过三维照片重建 (3D-PR) 技术测量小沟几何的最新进展，该技术允许人们通过低空航空和地面摄影测量获得数字地形模型 (DTM)。实际上，使用流动阻力方程估计流速和深度也需要有关小沟几何形状的信息。

然后，报告了 1984 年至 2022 年进行的相关实验室和实地调查的结果。特别是，考虑到由于颗粒阻力、形态阻力（阶梯池）和沉积物输送引起的细沟流动阻力的不同组成部分的研究，进行了审查。此外，还简要提到了等于 18% 的坡度阈值，用于确定水力和泥沙输送变量的差异。

此外，还进行了一项分析，以证明理论的小溪流阻力定律已被错误地作为虚假相关性的示例提出。

最后，强调了对细流水力学的一些研究需求。